A digital audio/video data processing unit such as a digital television decoder or “set top box” generally includes a main module which contains several or all of the essential functionalities for the decryption, the decompression and the interfacing of the audio/video data received from a cable, wireless or satellite broadcasting network or from a public telephone line.
The module comprises amongst others a central unit managing a deciphering unit, a decompression unit and different interfaces that allow the module to communicate with different external peripherals associated to the decoder such as a security module or multimedia equipments. The majority of these elements are connected to a central bus inside the module. This bus also links external memories for the temporary storage of work data or for the storage of audio/video contents for further use.
The decryption process of the audio/video data stream coming from the broadcasting network performed by the deciphering unit, needs intermediate data storage in a memory with a relatively large capacity separated from the main module. This stored data, which can be in the form of audio/video data blocks, is then decompressed before its routing towards an output interface that allows its visualization on a television screen or its recording on a magnetic or optical recording medium such as a hard disk or a compact disk (CD Compact Disc, DVD Digital Versatile Disc or other).
The presence of this decrypted data in a separate memory constitutes a protection failure of the compressed audio/video data. In fact, this data can be diverted and collected in order to be processed and distributed by an external third party system with the aim of abusive exploitation.
Moreover, data stored for future use, for example in a hard disk, can also be recovered and reproduced without the supplier's authorization, thus violating the regulations on copyright.
The document U.S. Pat. No. 6,286,103 describes a decoder of encrypted audio/video data wherein a security module deciphers the data and re-encrypts it before transmitting to the decoder. According to an embodiment, the transmission between the security module and the decoder is secured by way of a session key created from a random number generated by the decoder and encrypted with a key issued from the security module. This number, being combined with the data decryption key, varies frequently allowing preventing or rendering more difficult the decryption by a third party having obtained said decryption key fraudulently.
The document WO00/57636 describes various applications of security modules in form of cards personalized according to the categories of users. In particular, in a network of multimedia contents decoders, the deciphered data are re-encrypted. The decryption is possible only when the decoder is equipped with a duly authenticated card containing parameters specific to the user. In an embodiment, a card is authenticated by using a random number generated by the decoder and transmitted to the card. The latter encrypts the random number with a private key and resends it to the decoder, which decrypts it with the corresponding public key. A comparison between the generated number and the received number allows verifying the authenticity of the card.